1. Field of the Invention
The present invention relates to a process for fabricating integrated circuit (IC) sensors and/or actuators and, more particularly, to a process for fabricating electrically isolated silicon "islands" which interface with silicon "beams". The islands and beams are constructed in a manner which completely electrically isolates the islands and beams from the IC substrate, has a low parasitic capacitance, and may be compatible with a complementary metal oxide semiconductor (CMOS) process.
2. Discussion of Related Art
Currently, integrated circuit (IC) sensors and actuators, such as accelerometers, pressure sensors, and gyroscopes are manufactured using either a bulk micromachining process or a surface micromachining process. "Bulk" micromachining refers to structures formed by deep anisotropic etching. "Surface" micromachining refers to structures formed from thin film layers deposited or grown on the surface of a substrate. Surface micromachining has advantages over the previous bulk micromachining process of fabricating IC sensors and actuators because it permits smaller devices and may by integrated with other circuits on an IC.
Surface micromachining typically involves etching in a silicon substrate deep trenches between 10 microns and 100 microns deep. The resulting silicon structures (called "beams") are partially released (i.e., detached) from the silicon substrate by known processes such as wet or dry etching. This deep trench technology is described, for example, in Klaassen, et al. "Fusion Bonding and Deep Reactive Ion Etching: A New Technology for Microstructures," Transducers '95, Stockholm, Sweden, 1995. The contents of this article are incorporated herein by reference.
FIG. 1 is a top view of a typical micromachined sensor or actuator structure 50. (FIG. 1 shows four thermal actuators). The sensor/actuator 50 includes a substrate 52 from which a number of beams 54 are cantilevered via deep trenches 56 formed in the substrate 52. The beams 54 are connected to the substrate 52 at anchor portions 58.
One problem associated with the beams is electrically isolating them from the substrate. Several known techniques are used to address this problem. A first technique for electrically isolating sensor/actuator beams from an IC substrate is to cover the beams with an electrical insulator such as a silicon dioxide film. A thin film of aluminum (Al) or other suitable metal is then deposited on the insulator. The metal operates as an electrically conductive portion of the device and the beams, which are electrically isolated by the surrounding silicon dioxide film, act only as a mechanical structure. One drawback of this technique is that the metalization causes metal particles to "creep", thus the sensor or actuator is vulnerable to degraded performance due to time and temperature. This structure is also subject to a high parasitic capacitance to the IC substrate.
A second technique for electrically isolating sensor/actuator beams from an IC substrate is to isolate the beams from the substrate by leaving a cavity beneath the beams using a silicon bonding technique, as described in the Klaassen, et al. article referred to above. The beam anchors (where one end of beams are connected to the substrate) are electrically isolated from the substrate with an intermediate oxide layer grown during silicon wafer bonding.
A third technique for electrically isolating sensor/actuator beams from an IC substrate is to use impurity diffusion to isolate the beams. This technique is prone to parasitic capacitance and leakage current.
A fourth technique for electrically isolating sensor/actuator beams from an IC substrate is to fusion bond the beams to a layer of electrically isolating oxide which separates the beams from the substrate. Although this provides effective electrical isolation, it is difficult to provide planar surfaces for further processing. This is because the beams extend above the IC substrate by tens of microns. Running metalization to connect them to the IC may be difficult due to the "steps" in the surface. The beams are surrounded by trenches and diffusion isolation for planar leadouts may cause electrical leakage and high parasitic capacitance at the trench/diffused junction interface.
Therefore, it is an object of the present invention to provide a process for manufacturing IC sensors and/or actuators that completely electrically isolates the sensor's beams from the substrate.
It is another object of the present invention to provide a process for manufacturing IC sensors and/or actuators that has low parasitic capacitance.
It is yet another object of the present invention to provide a process for manufacturing IC sensors and/or actuators that is compatible with CMOS processes.